Opto-Acoustic Imaging

1. Opto-Acoustic Imaging 台大電機系李百祺

2. Conventional Ultrasonic Imaging • Spatial resolution is mainly determined by frequency. Fabrication of high frequency array transducers is complicated: - l/2 pitch between adjacent channels. - l/2 thickness of the piezoelectrical material. • Both are at the order of 10mm. • Other complications include bandwidth, matching, acoustic and electrical isolation, and electrical contact.

3. Conventional Ultrasonic Imaging • Contrast resolution is inherently limited by differences in acoustic backscattered properties. • Low contrast detectability is further limited by speckle noise. • A new contrast mechanism is desired. One such example is the elastic property.

4. Opto-Acoustical Imaging • Acoustic waves can be generated and detected using optical methods. • Size limitations of conventional piezoelectrical materials can be overcome using laser techniques. • Sensitivity and efficiency are critical issues.

5. Optical Generation of Acoustic Waves (I) • Absorption of optical energy produces thermoelastic waves. • A membrane with proper thermoelastic properties can be used to transmit acoustic waves.

6. Optical Generation of Acoustic Waves (II) • Optical absorption can be viewed as a contrast mechanism (i.e., different tissues have different absorption coefficient, therefore produce acoustic waves of different amplitudes). • Detection of such signals is still determined by inherent acoustic properties.

7. Optical Detection of Acoustic Waves • Movement of a surface due to acoustic waves can be measured by using optical interference methods. • Size of such detectors is determined by the laser spot size. • Laser spot size can be a few microns, thus acoustic imaging up to 100MHz is possible. • Remote detection.

8. High Frequency Opto-Acoustic Imaging • Opto-acoustic phased array at very high frequency (>=100MHz). • Resolution at a few microns. • Rapid scanning. • Synthetic aperture imaging. • Compact.

9. Opto-Acoustical Imaging of Absorption Coefficient • Rapid growing cancer cells often need extra blood supply. • High blood content is related to high optical absorption coefficient. • High optical contrast can be combined with low acoustic scattering and attenuation.

10. Basics of Laser Operations • Light Amplification by Stimulated Emission of Radiation: a method to generate high power, (almost) single frequency radiation with wavelength ranging from 200nm to 10mm. • Visible light is from 400 to 700 nm.

11. Output beam Lasing medium Fully reflecting mirror Partially transmitting mirror Basics of Laser Operations • Two basic components: a resonator (cavity) and a gain medium (pump). • Resonator: cavity length is half wavelength.

12. E2 Lasing transition Pump E1 E0 Basics of Laser Operations • Two basic components: a resonator (cavity) and a gain medium (pump). • The gain medium can be gas, liquid or solid. It provides stimulated emission.

13. Characteristics of Laser • Monochromaticity. • Coherence. • Directionality. • High intensity.

14. High Frequency Ultrasound Imaging Using Optical Arrays

15. Ultrasonic Array Imaging • Benefits: • Dynamic steering and focusing. • Adaptive image formation. • Requirements: • Element spacing at l/2. • Large numerical aperture. • Wide bandwidth.

16. High Frequency Ultrasonic Array Imaging (100MHz or greater) • Complications: • Element spacing is 7.5mm at 100MHz. • Acoustic matching. • Electrical contact. • Acoustic and electrical isolation. • Interconnection.

17. High Frequency Ultrasonic Imaging Using Optical Arrays • Generation: instantaneous absorption ↑ temperature change ↑ stress ↑ acoustic wave. • Detection: • Confocal Fabry-Perot interferometer. • Ultrasonic motion ↑ phase modulation ↑ Doppler shift.

18. High Frequency Ultrasonic Imaging Using Optical Arrays • Precise control of position and size. • Synthetic aperture with rapid scanning. • Element size and spacing at the order of a few mm’s.

19. High Frequency Ultrasonic Imaging Using Optical Arrays • Large bandwidth (both transmit and receive). • Transmission using fibers (low loss and high isolation). • Non-contact and remote inspection.

20. Detection System Set-up

21. Image Formation • Synthetic Aperture. • 1D or 2D aperture. • Image plane is defined by scanning of the laser beam. • Side-scattering vs. back-scattering.

22. Wire Images Using a 1D Array

23. Wire Images Using a 1D Array

24. Cyst Images Using a 2D Array

25. Cyst Images Using a 2D Array

26. Optical Biopsy Probe

27. Discussion • Optical generation of acoustic waves. • Improved receive sensitivity by active optic detection (displacement changes the laser cavity length). • Higher frequencies.

28. Sensitivity of Laser Opto-Acoustic Imaging in Detection of Small Deeply Embedded Tumors

29. Motivation • Develop an imaging technique for low contrast, small tumors. • Optical contrast mechanism (between normal tissue and tumor): • Absorption: blood content, porphyrins. • Scattering: micro-structures.

30. Advantages • High optical contrast in the NIR range. • Low acoustic scattering and attenuation. • Fig. 1.

31. Thermo-elastic pressure waves • Absorption -> Temperature rise -> Pressure rise. • Under the condition of temporal stress confinement, i.e., insignificant stress relaxation during laser pulse. • t<d/cs. • Half-wavelength resonator.

32. Materials and Methods • Fig. 2. • Q-switched Nd:YAG laser: • l=1064 nm. • 1/e level 14 ns. • 0.2 J/cm2 (ANSI 0.1-0.2). • PVDF 5MHz bandwidth transducer, lithium-niobate 100MHz transducer (?).

33. Materials and Methods • Breast phantom 1: • Normal tissue: gelatin+polystyrene spheres (900nm) or milk for scattering. • Tumors: bovine hemoglobin, 2-6mm. • Breast phantom2: • Bovine liver (3mmX2mmX0.6mm). • Placed between chicken breast.

34. Results • Fig 4. To Fig. 6. • Fig. 7 to Fig. 8: Simulations based on existing measurements (2mm sphere at 60mm depth). • Wavelet transform for noise reduction.

35. Complications • Acoustic attenuation not present in gelatin phantoms: • Typically 0.5dB/cm/MHz. • The smaller the tumor, the higher the attenuation. • Tissue inhomogeneities exist in breast tissue. • Receiver center frequency and bandwidth. • Lateral resolution vs. axial resolution.

36. Depth Profiling of Absorbing Soft Materials Using Photoacoustic Methods

37. Motivation • Characterize absorbing properties and detect boundaries of layered absorbing materials, such as skin. • Acoustic waves are generated by rapid deposition of laser energy into optically absorbing materials – thermoelastic effects. • Pressure(R) -> Absorption Coefficient(R).

38. Materials Under Investigation • India Ink (photo-stable absorber) in water solutions and acrylamide gels. • India-ink stained biomaterials. • Layered absorbing media using acrylamide gel.

39. Theory • Thermoelastic process: stress confinement. (eq.1) • Highly attenuating materials: Beer’s law. Optical scattering, acoustic attenuation are ignored. (eq.2) • Near field condition for plane wave assumption. (eq.3) • Fig.1 and Fig. 2.

40. Materials and Methods • Fig. 3. • Laser spot size: 3-5mm. • Laser radiant exposure: 0.2-1.2 J/cm2. • Lithium niobate transducer protected by a quartz window (800ns delay).

41. Materials and Methods • Calibration using known concentration of India ink in solution (calibration factor mV/bar). • India ink with absorption coefficient 2650cm-1 was used to make absorbing solutions in the range from 15 to 188cm-1.

42. Materials and Methods • Acrylamide gels were used to create layers of absorbers as thin as 90mm. • Porcine aorta was processed such that only the elastin layer was used. • The intimal surface was stained by India ink. The opposite surface was in contact with the piezoelectric transducer.

43. Materials and Methods • Fig. 4. • Determination of absorption coefficient based on Beer’s law. Eqs. 7-11.

44. Results • Fig. 5 – Fig. 11.

45. Discussion • Gel layer resolution is affected by acoustic attenuation and transducer bandwidth. • Stain diffusion of elastin biomaterial. Eq. 13. • The scattering coefficient may not be ignored in practice. • Potential application: laser-tissue welding (measuring the chromophore deposition and temperature profile).